(Ri Joo Kim)
1
(Han Gyeol Jeong)
1
(Ye Ji Son)
1
(So Yeon Si)
1
(Sung-Soo Ryu)
3
(Sang Ki Ko)
2
(Hyun Seon Hong)
1*
Copyright © 2023 The Korean Institute of Metals and Materials
Key words(Korean)
Al2O3 powder, densification, Microstructure, Insulator
1. INTRODUCTION
During semiconductor fabrication processing, materials exposed to plasma undergo radical
reactions while simultaneously being impacted by positive ions, resulting in plasma
etching and the production of various contaminating particles [1-4]. For that reason, plasma-resistant materials must be used for semiconductor processing
equipment. The demand for alumina (Al2O3) material, which has near-perfect purity, high density, high plasma resistance, and
few surface defects, has grown, since the use of plasma has increased with the scaling
of semiconductor processes [5-8]. When using high voltage at a specific high frequency in the semiconductor etching
process, materials are needed which have stable electromagnetic wave transmission
and minimal leakage current, to withstand the high voltages. Alumina (Al2O3), with its excellent electrical properties and high density, is a good candidate
for this application. Currently, alumina (Al2O3) is often utilized in the insulators used for the electrostatic chuck (ESC) bottom-mounted
substrate, in the etching and chemical vapor deposition processes used to fabricate
semiconductors and displays [9, 10].
Al2O3 is the most widely used ceramic material, and is useful in the semiconductor and
battery industries. Al2O3 can be applied to different material parts because of its high strength, thermal
and electric insulating properties, and corrosion resistance. High density alumina
components are obtained by compressing Al2O3 powder and sintering it at 1700 °C or higher. However, the metals used in various
industries have melting points (e.g., Ti: 1610 °C, stainless steel: 1400 °C) which
are lower than the generally required sintering temperature of Al2O3. In efforts to sinter Al2O3 at a lower temperature, various sintering methods, such as isostatic pressure, hot
press, and gas pressure sintering, have been developed [12-15]. However, these kinds of sintering processes require expensive equipment and involve
hazardous processes, while also being limited by the particle size of the manufactured
ceramics. As an alternative to introducing a new sintering process, the sintering
temperature can be lowered by using a sintering agent. Several additives have currently
been used as sintering aids. In general, when a sintering additives such as NiO, SiO2, MnO, FeO, MgO, or TiO2, have been used, a high sintering density can be achieved, even at a relatively lower
temperature, and their mechanical properties can be enhanced by controlling their
microstructure [16-20].
Although there have been related studies on the doping of porcelain with BaTiO3, which resulted in a low sintering density of less than 3.00 g/cm3, there are still few studies of both the microstructure and sintering behavior when
adding additives to Al2O3 for insulating materials [21]. Therefore, in this study, basic alumina sintering behavior was studied, to develop
an alumina insulator. The effect of additives and sintering temperature on sintering
behavior was analyzed, and the effects of microstructure changes on density and mechanical
properties with the addition of TiO2 as a sintering agent to Al2O3, were investigated.
2. EXPERIMENTAL PROCEDURES
Samples used in experiments were Al2O3 (AES-11, Sumitomo Chemical Co., Ltd., Japan, 99.9%, D50 0.43 µm) and TiO2 (Shanghai SMEC Enterprise Co., Ltd., China, 99.5%, D50 0.70 µm, Batch no. 19090340245). The organic binder used was PEG 8000 (Duksan, South
Korea). Figure 1 shows a scanning electron microscope image of the primary particles, which are hundreds
of nanometers in size, and secondary particles consisting of aggregated primary particles
of the Al2O3 raw material powder, with TiO2 powder added.
To produce sintering specimens, 0.05, 0.5, 1.0, 3.0, and 5.0 wt.% TiO2 were combined and mixed into Al2O3 powder. The total mixed powder was 30 g by weight and was ball milled at 200 rpm
for 24 h in wide-mouth bottles (NA-2104-0008, 250 ml) containing 200 g of Al2O3 balls, 70 ml of ethanol, and 0.3 g of organic binder (PEG 8000). The postmix slurry
was dried in an oven at 70 °C, and the dried powder was uniaxially pressed at 60 MPa
for 1 min. The specimens were uniformly molded int°Cylinders with a diameter of 20
mm and thickness of 2 mm, and then sintered at 1400, 1500, or 1600 °C for 2 h.
The shrinkage rate was calculated by comparing the diameter and thickness of the heat-treated
sintered bodies at each temperature and before sintering, while the density and porosity
were analyzed using the Archimedes principle in pure water. Field emission scanning
electron microscopy (FE-SEM; JSM-7500F, JEOL Ltd., JAPAN) was used to analyze the
microstructure of the sintered bodies. X-ray diffraction (XRD; D8 Focus, Bruker, Germany)
was performed to determine the crystal structure of the sintered bodies. Pure Al2O3 samples sintered at 1400, 1500, and 1600 °C were analyzed t°Clearly understand the
effect of TiO2 as a sintering additive. Before FE-SEM analysis, the sintered surface was polished,
and underwent thermal etching at 1350 °C for 2 h. XRD analysis was performed under
the conditions of an angle range of 20–80°, scan rate of 5o/min, 40 kV, and 40 mA.
In addition, the Vickers hardness of the sintered bodies was analyzed using the Vickers
hardness test. Hardness was measured with a load of 3 kg (810-165K, Mitutoyo, Japan).
3. RESULTS AND DISCUSSION
3.1 TiO2 content and density change after sintering
Figure 2(a) shows the relative density of the alumina-sintered bodies heat-treated at various
temperatures according to TiO2 concentration. The relative densities were calculated using the theoretical density
values of Al, O and Ti, which were 3.95 cm³ for Al2O3 and 4.23 cm³ for TiO2, respectively. The maximum relative density of 99.6% was observed in the specimen
with 0.05 wt.% TiO2, and the lowest relative density was 96.6%, observed in the specimen with 5.0 wt.%
TiO2. The change in density showed a similar trend at a sintering temperature of 1500
°C. In contrast, when sintered at a relatively low temperature of 1400 °C, the relative
density of the 0.05 wt.% TiO2 specimen was below 92%, an extremely low value compared with the specimens sintered
at higher temperatures. However, when 0.5 wt.% or more of TiO2 was added, the density rapidly increased and exhibited values similar to those at
higher temperatures.
The change in density with increased TiO2 content can be explained as follows. When a small amount of sintering agent is added,
the TiO2 is thought to promote the sintering of Al2O3, even at lower temperatures, which crystallizes quickly, increasing the specimen’s
density as a result. When the amount of added TiO2 exceeds 0.5 wt.%, the relative density gradually decreases. This is thought to occur
because of over-sintering and generation of a secondary phase, due to the presence
of TiO2 [22,23]. When the dopants were added to Al2O3, the TiO2 resulted either in atomic segregation at alumina-alumina grain boundaries, or in
the formation of various secondary crystalline phases, with a profound influence on
the densification of the Al2O3 ceramic. This could also result in lower interpore spacing, generating a large number
of pores, degrading the density of the specimens [8,9].
Figure 2(b) shows the changes in relative density according to the sintering temperature with
TiO2 content. Observing the density changes of the specimen with 0.05 wt.% of TiO2, the relative density value was 91.7% when sintered at 1400 °C, significantly lower
than the relative density values of samples sintered at 1500 °C and 1600 °C, 98.2%
and 99.6%, respectively.
When the alumina was thermally treated at 1400 °C, the sample was not sufficiently
sintered, due to the low temperature. Raw alumina powder due to incomplete sintering
was observed in the microstructure analysis. When pure alumina powder with no sintering
was heat-treated at 1400 °C, the sintering pattern was the same as that seen with
0.05 wt.% TiO2. However, when more than 0.5 wt.% of TiO2 is added, sintering became active and proceeded at 1400 °C, producing a higher relative
density value.
As described for the case of 1400 °C in this experiment, Al2O3 sintering did not progress completely at this low sintering temperature. However,
at a temperature of 1500 °C or higher, sintering proceeded well regardless of the
sintering agent content, leading to densification and high relative density values.
Moreover, as the content of the sintering agent increased to 0.5 wt.%, the density
tended to decrease slightly. Therefore, in terms of density, it was concluded that
the optimal sintering temperature and TiO2 content were between 1500 °C–1600 °C and 0.05 wt.% –0.5 wt.%, respectively.
Figure 3 shows the shrinkage rate according to the concentration of the TiO2 additive and heating temperature; the trends at 1500 and 1600 °C are similar but
differed from the case at 1400 °C. the specimens with up to 5.0 wt.% TiO2 sintered at 1500 °C and 1600 °C shrunk by 3.0% and 3.8%, respectively, compared to
pure Al2O3. As the TiO2 content increased, the decrease was 3.4% on average, as the trends at the two temperatures
were similar. However, at 1400 °C, the shrinkage rate increased to a value similar
to that of specimens sintered at the other temperatures when 0.5 wt.% or more was
added, because the TiO2 did not act as a sintering agent at a lower sintering temperature of 1400 °C.
In summary, the relative density and shrinkage rate according to TiO2 content exhibited the same change trend, and different tendencies were observed when
the sintering temperature was varied. Density and shrinkage values were lower at 1400
°C, when 0.05 wt.% or less TiO2 was added. However, when a content of 0.5 wt.% or more was added, TiO2 acted like a sintering aid, and the density and shrinkage rate values rapidly increased.
At 1500 °C and 1600 °C, high density and shrinkage values were maintained at all contents,
and showed similar patterns. However, when the TiO2 content exceeded 1.0 wt.% or more, it gradually decreased.
3.2 Crystalline structure according to TiO2 content and sintering temperature
XRD analysis was performed to analyze the phase of the sintered bodies based on the
TiO2 content and sintering temperature. Figure 4 shows the XRD intensity peaks for pure Al2O3 and those of the Al2O3 specimen containing 0.05 wt.% TiO2 sintered at 1400, 1500, and 1600 °C. In both figures, at 1400 °C the peaks are relatively
low in intensity and broad in width, which suggests low crystallinity. However, when
the sintering was conducted at a temperature of 1500 °C or higher, the peaks were
narrow and high, confirming there was an increase in crystallinity with increasing
sintering temperature.
Figure 5 is the result of analyzing the crystal structure using XRD for the sintered samples
at 1400 °C. The peaks are compared for the pure Al2O3 specimen and the specimens with 0.05–5.0 wt.% TiO2 added. The peaks for the pure Al2O3 specimen and that with 0.05 wt.% TiO2 are relatively low in height and width, suggesting low crystallinity, while specimens
with up to 0.5 wt.% TiO2 were confirmed to have high crystallinity, as evidenced by the narrow and high XRD
peaks.
Thus, when sintering is performed at 1400 °C, the addition of 0.5 wt.% TiO2 results in good sintering performance as well as an increase in crystallinity and
density. In the specimen with 3.0 wt.% or more of added TiO2, a peak corresponding to Al2TiO5 was observed, which is a secondary phase. The peak intensity of the secondary phase
is extremely weak; thus, the proportion of this secondary phase is not high. However,
it is believed that the formation of the secondary particles with increasing TiO2 content results in a lowering of density. In other words, the formation of the secondary
phase during sintering may affect the density. Because the secondary phase peak in
this study is extremely weak, it was impossible to quantitatively analyze its effect
on density.
3.3 Microstructure changes according to TiO2 content and sintering temperature
Figures 6(a)–(c) and Table 1 show the microstructure of pure Al2O3 samples without sintering aids, sintered at 1400, 1500, and 1600 °C, respectively.
After sintering, the average particle size increased to 1.2 μm at 1400 °C, 3.2 μm
at 1500 °C, and 4.1 μm at 1600 °C, with no abnormal grain growth.
The microstructure of the pure Al2O3 sample sintered at a temperature of 1400 °C was unique. The Al2O3 particles used as raw materials were not completely sintered, as shown in the area
marked by ‘A’ in Figure 6(a). Figures 6(d)–(f) show the microstructures of the 0.05 wt.% TiO2 -Al2O3 samples sintered at 1400, 1500, and 1600 °C, respectively. As in the pure alumina
specimens, the grain size grew as the sintering temperature increased, and sintering
did not occur properly at 1400 °C due to the lower sintering temperature.
Figures 7(a)–(d) show SEM images of the 0.5, 1.0, 3.0, and 5.0 wt.% TiO2 added samples, respectively, with sintering at 1400 °C. When sintered at this temperature,
the average particle sizes were 5.5, 4.2, 4.4, and 3.9 μm, respectively, showing a
slight decrease in particle size for the samples with more than 1.0 wt.% of TiO2.
Figures 7(e)–(h) show SEM images of samples with 0.5, 1.0, 3.0 and 5.0 wt.% TiO2 added, respectively, with sintering at 1500 °C. Compared to the samples sintered
at 1400 °C, the grain growth is relatively even and regular. The average particle
sizes for each sample are 11.2, 9.9, 7.4, and 6.8 μm, respectively, and as observed
at 1400 °C, the particle size was largest at 0.5 wt.% and decreased at higher TiO2 content. This can be attributed to the generation of the secondary phase, with increased
TiO2 content. Because TiO2 helps Al2O3 grain growth, the particle size for specimens at levels below 1.0 wt.% of TiO2 increased, but the particle size for specimens with more than 1.0 wt.% TiO2 gradually decreased, because the generated secondary phase suppresses particle growth
and hinders densification. Figures 7(i)–(l) are SEM images of samples sintered at 1600 °C. The average particle sizes were 11.5,
8.9, 7.7, and 6.9 μm, respectively, all larger than those observed when sintered at
1400 and 1500 °C.
Specimens containing 5.0 wt.% of TiO2 were sintered at 1400, 1500, and 1600 °C, and the results of energy dispersive spectroscopy
(EDS) analysis are presented in Figure 8. The Ti was observed to be uniformly distributed throughout the specimen with no
clear segregation phenomenon.Table 2.
3.4 Changes in hardness with sintering agent concentration and temperature
Figure 9 shows the changes in hardness with varying TiO2 content and at different sintering temperatures. Vickers hardness was measured 5
times for each specimen. The error bars are used to indicate the range of standard
deviation in the results. At all sintering temperatures, the Vickers hardness values
increased as the amount of TiO2 increased, showing the highest value at 1.0 wt.%. The average Vickers hardness value
of commercial sintered alumina is reported to be 1300 HV [11]. In this study, the Vickers hardness value of the sample sintered at 1600 °C showed
an average value of 1400 HV. The maximum hardness value was 1729 HV for 1.0 wt.% TiO2 sintered at 1600 °C. When TiO2 was not added, the Vickers hardness values varied greatly with sintering temperature,
but were similar at 1.0 wt.% TiO2. The largest increase or decrease in the hardness value occurred in a sintered specimen
at 1400 °C. The hardness of the specimen with no added TiO2 sintered at 1400 °C was significantly lower than the hardness values under the other
conditions.
Comparing the results of hardness testing with density measurements revealed that
they followed a similar trend. Depending on the TiO2 content, the hardness value reached a maximum at 0.5–1.0 wt.% and a minimum at 3.0–5.0
wt.%. The XRD analysis showed that a secondary phase of Al2TiO5 was formed when TiO2 was added at 3.0 wt.% or more. The hardness of the sintered body was considered to
have decreased because of the presence of this secondary phase. In addition, the density
value of the specimen with low TiO2 content, sintered at 1400 °C, was low. The lower relative density and hardness value
are likely caused by the relatively low sintering temperature and insufficient densification.
Using these results, the requiring sintering temperature and the TiO2 content required to manufacture alumina-sintered bodies with appropriate mechanical
characteristics for an insulator were obtained.
4. CONCLUSIONS
In this study, Al2O3 was sintered while adding TiO2 as a sintering agent, and the following conclusions can be made after analyzing the
densification and microstructure characteristics of the sintered bodies thus prepared.
When TiO2 was added to pure Al2O3 at contents between 0.05–1.0 wt.% and sintered at a temperature of 1500 °C, the average
relative density value was 98% or more, indicating that the addition of 1.0 wt.% TiO2 and below promotes densification of the sintered body.
Microstructure analysis using SEM showed that TiO2 acted as a sintering aid and allowed Al2O3 to be sintered at a lower temperature than originally required. Also, the particle
size of the sintered body increased with the addition of up to 0.5 wt.% TiO2, and the particle size gradually decreased when the added amount exceeded 1.0 wt.%.
The decrease in particle size can be explained by densification interference due to
an increase in secondary phase generation when 1.0 wt.% or more TiO2 was added. This secondary phase generation was confirmed through XRD analysis.
Like the density value, the hardness of the sintered body reached maximum value (1638
HV on average) when the range of added TiO2 was.5 to 1.0 wt.%, and the hardness increased as the relative density value increased,
indicating that the internal densification of the specimens affected mechanical properties.
This study confirmed that changes in sintering temperature and TiO2 content affected the microstructure and mechanical properties of the sintered Al2O3 bodies. In addition, TiO2 was observed to act as a significant sintering aid within the range of 0.05 to 1.0
wt.%, and produced a high-hardness, dense Al2O3 insulator.
Acknowledgements
This work was supported by the Technology development Program (S3152379) funded by
the Ministry of SMEs and Startups (MSS, Korea)
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Figures and Table
Fig. 1.
SEM image of the raw materials (a) Al2O3 powder and (b) TiO2 powder
Fig. 2.
Relative density of densified Al2O3 plotted as functions of (a) TiO2 concentration and (b) sintering temperature
Fig. 3.
Shrinkage results of Al2O3 samples with different TiO2 contents
Fig. 4.
XRD spectra for (a) pure Al2O3 specimens and (b) 0.05wt% TiO2 doped Al2O3 specimens, both sintered at 1400 °C, 1500 °C, and 1600 °C
Fig. 5.
XRD spectra for the Al2O3 specimen with different TiO2 contents, sintered at 1400 °C
Fig. 6.
Polished cross-sectional SEM images of pure Al2O3 sintered at (a) 1400 °C (b) 1500 °C (c) 1600 °C and Al2O3 with the addition of 0.05 wt% TiO2 sintered at (d) 1400 °C (e) 1500 °C and (f) 1600 °C
Fig. 7.
Polished cross-sectional SEM images of Al2O3 sintered at 1400 °C with the addition of (a) 0.5wt%, (b) 1.0wt%, (c) 3.0wt% and (d)
5.0wt% TiO2, at 1500 °C with the addition of (e) 0.5wt%, (f) 1.0wt%, (g) 3.0wt% and (h) 5.0wt%
TiO2, and at 1600 °C with the addition of (i) 0.5wt%, (j) 1.0wt%, (k) 3.0wt% and (l) 5.0wt%
TiO2
Fig. 8.
Elemental maps (EDS) of Al in (a) 1400 °C sintered specimen, (b) 1500 °C sintered
specimen, (c) 1600 °C sintered specimen and Ti in (d) 1400 °C sintered specimen, (e)
1500 °C sintered specimen, (f) 1600 °C sintered specimen
Fig. 9.
The variation in Vickers hardness with TiO2 concentration and sintering temperature
Table 1.
Average particle size of pure and 0.05 wt% TiO2 added specimens sintered at 1400 °C, 1500 °C, 1600 °C
|
Sintering temperature (°C)
|
TiO2 content (wt%)
|
Particle size (µm)
|
|
1400
|
Pure
|
1.16
|
|
1400
|
0.05
|
1.50
|
|
1500
|
Pure
|
3.23
|
|
1500
|
0.05
|
4.85
|
|
1600
|
Pure
|
4.10
|
|
1600
|
0.05
|
6.00
|
Table 2.
Average particle sizes of specimens with different sintering temperature and TiO2 content
|
Sintering temperature (°C)
|
TiO2 content (wt%)
|
Particle size (µm)
|
|
1400
|
0.5
|
5.50
|
|
1.0
|
4.23
|
|
3.0
|
4.40
|
|
5.0
|
3.93
|
|
1500
|
0.5
|
11.20
|
|
1.0
|
9.85
|
|
3.0
|
7.40
|
|
5.0
|
6.80
|
|
1600
|
0.5
|
11.54
|
|
1.0
|
8.93
|
|
3.0
|
7.71
|
|
5.0
|
6.93
|